1. Field of the Invention
This invention relates to an improved process for the production of uretdione group-containing compounds by dimerizing at least a part of the isocyanate groups of organic isocyanates, particularly of organic polyisocyanates with aliphatically-bound isocyanate groups, using new dimerization catalysts, which have both a catalyst component as well as a co-catalyst component, the uretdione group-containing compounds obtained according to this process and the use of the uretdione group-containing compounds based on organic polyisocyanates, obtained according to this process, in the production of polyurethane plastics materials.
2. Description of the Prior Art
While various processes are known for dimerizing aromatic isocyanates (see e.g. Saunders and Frisch, "Polyurethanes-Chemistry and Technology", Part I, 1962, p. 61 ff.), suitable processes for dimerizing isocyanates with aliphatically-bound isocyanate groups have so far hardly been described.
The longest-known process, according to which uretdione group-containing products are obtained starting from aliphatic isocyanates, is described in DE-OS No. 1,670,720. In this process trialkylphosphines in relatively high concentrations of up to 2% by weight are used as dimerization catalysts. Pure uretdiones are not produced, but rather mixtures of uretdiones and isocyanurates.
A recently-known process for the production of a practically isocyanurate-free isocyanatouretdione based on aliphatic isocyanate is shown by the process described in DE-AS No. 3,030,513 for dimerizing 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexylisocyanate (isophorone diisocyanate). In this process, the amides of the phosphoric acids (aminophosphines), already described in U.S. Pat. No. 3,290,288 as dimerization catalysts for aromatic isocyanates, are used as dimerization catalysts. The isocyanato-uretdiones based on isophorone diisocyanate obtained in this manner are indeed practically free of isocyanurates, but the disadvantage in this process is that relatively high catalyst quantities are necessary for a sufficiently fast reaction; for example 2% by weight of tris-(dimethylamino)-phosphine are required in order to achieve an NCO-conversion of from 15 to 20% within from 10 to 30 hours.
This circumstance not only reduces the economy of the process due to the high catalyst concentration and the long reaction time, but the quality of the reaction products is also influenced in a disadvantageous manner.
A substantial disadvantage of the dimerization process described in DE-AS No. 3,030,513 is, moreover, the restricted use of this process.
Thus, for example, the attempt to react 2-methyl-1,5-diisocyanatopentane to produce practically isocyanurate-free products fails. As shown in DE-OS No. 3,227,779, the 2-methyl-1,5-diisocyanatopentane (predominantly in admixture with up to 12% by weight of 2-ethyl-1,4-diisocyanatobutane) can be dimerized in the presence of tris-(dimethylamino)-phosphine even under very mild conditions, but notable quantities of isocyanurates are also produced.
Furthermore, the process used in DE-OS No. 3,030,513 for the selective dimerization of isophoronediisocyanate also fails in the attempt to dimerize in a practically isocyanurate-free manner 1,6-diisocyanatohexane, which is of great technical importance. Products with a more or less high isocyanurate content are always obtained. This is described, for example, by M. I. Bakhitov et al (Vysokomol. Soedin., Ser. B (1981), 23 (9), pp. 680-682: see also Chemical Abstracts 96: 20 504 c) in which 1,6-diisocyanatohexane is reacted in the presence of tris-(diethylamino)-phosphine to produce isocyanurate group- and uretdione group-containing oligomers (see also SU-PS 907014, Chemical Abstracts 97: 24 371 a). As is moreover described by M. I. Bakhitov et al (Vysokomol Soedin., Ser. B (1983), 25 (11), pp. 830-833; see also Chemical Abstracts 100: 86 171 z), n-butylisocyanate and phenylisocyanate are also reacted during treatment with tris-(diethylamino)-phosphine to produce reaction products, which always contain troublesome quantities of isocyanurates.
The presence of isocyanurate groups is undesirable for a number of uses of dimeric isocyanates since isocyanurate group-containing uretdiones or isocyanatouretdiones are not completely reconvertible to the starting isocyanates. This impairs their use, for example, in uses requiring reactivation of isocyanate groups. Isocyanurate group-containing isocyanatouretdiones moreover have a relatively high viscosity, due to their branched structure, which substantially reduces their usefulness, particularly in polyurethane lacquers with a low solvent content.
It was thus an object of the invention to produce a new process for dimerizing organic isocyanates, which allows organic isocyanates to be dimerized with high selectivity, and which produces dimerization products of high quality, particularly regarding color.
This object could surprisingly be achieved by using the catalyst combinations described in more detail below as dimerization catalysts.